1. Field of the Invention
The present invention relates generally to semi-conductor assemblies and more particularly to high-power semiconductor assemblies for application in power supplies.
2. Description of the Prior Art
By the terms "disk-cell construction" or "disk-cell", there is meant in the present connection a disk-shaped semiconductor chip enclosed, with the interposition of electrodes and conducting plates, if necessary, in an insulating housing. Such high-power semiconductor components are generally known, such as, for example, from VDI-Zeitschrift 107 (1965), No. 34 - December, p. 1656, DT-PS 2,039,806. From the first-mentioned source it is further known how to clamp the disk cell between heat sinks of mushroom-like cross section, and, depending upon the desired current-carrying capacity, to provide ambient-air cooling, forced-air cooling, or liquid cooling.
From DT-PS 2,039,806 it is known how to use ductile, cup-shaped electrodes of silver. The open sides of the cup-shaped electrodes are disposed away from the main surfaces of the semiconductor chip. In this way, and by adjustment of the diametrical ratios and material thicknesses, a uniform pressure distribution and symmetry with respect to the central axis of the semiconductor component is obtained.
It is also to be noted in this connection that the aforementioned "essentially doubly symmetric arrangement" is to be so understood that, for example, for a power thyristor, a control terminal is to be provided which rather strongly affects the housing symmetry, if necessary, but which only slightly affects the pressure symmetry. Such disk-cells with a central control terminal (central gate) are also known (DT-OS 2,246,423).
The cooling of semiconductor components is described further in the "Silicon Rectifier Handbook" of Brown Boveri & Co., Baden (Switzerland) of 1971. FIG. 1 shows a known design of a power diode in disk form. There, 1 indicates the silicon semiconductor chip, 2 the electrodes of silver, for example, 3 the conducting plates consisting possibly of tungsten or molybdenum, and 4 the terminal electrodes or heat sinks consisting of copper, for instance.
It is known in this connection (DT-OS 1,944,181) how to avoid the use of any contact-making material between the parts 1 to 4, and to produce the contact for current passage and heat conduction by inward pressure on the element, for example, by clamping between two special heat sinks. The terminal electrodes can themselves be heat sinks or part of a heat sink or again be pressure-contacted with heat sinks. To the extent shown in FIG. 1, the component exhibits three heat-transmission resistances on each side which may be denoted by R.sub.v.sbsb.1 -R.sub.v.sbsb.3. They each arise at the transition from one material to the other, they are pressure, surface and material dependent, and the electrical output of the component essentially depends on their magnitudes.
It is additionally known (DT-PS 2,220,682) how to reduce these thermal resistances by introducing heat-conducting paste or grease between the pressure-contact materials. This sharply reduces the electrical conductivity, however. An improvement can be made in this respect by mixing metal powder with the heat-conducting paste or grease so as to increase the electrical conductivity. With prolonged operation of such semiconductor components, however, the heat-conducting paste or grease is decomposed by means of the current, leading to a rise in the thermal resistance.